The present invention relates to an optical recording medium, and more specifically, to an optical recording medium which allows data recorded in a recording film included in an information layer of the optical recording medium to be directly overwritten with new data as desired even when the data is kept for a long time.
As recording media for recording digital data in the related art, optical recording media such as CDs (Compact Disks) or DVDs (Digital Versatile Disks) have been widely used.
Among methods of recording data in the optical recording media, a recording method of modulating data to be recorded by a predetermined length as recording marks formed along tracks provided in the optical recording media is typically employed. For example, in a DVD-RW as an example of the optical recording medium allowing data to be rewritten by a user, recording marks of a length corresponding to values from 3T to 11T and 14T (“T” denotes one clock period) are used, and the recording marks are formed in a recording film included in an information layer along tracks provided in an optical recording medium so that data are recorded.
By doing so, when the recording marks are formed in the recording film included in the information layer of the data-rewritable optical recording medium to record data, a laser beam is irradiated on the recording film included in the information layer along the tracks provided in the optical recording medium, and a phase change material in a crystalline state contained in the recording film becomes amorphous, so that amorphous regions having a predetermined length is formed in the recording film included in the information layer, thereby using the formed amorphous region as the recording marks.
That is, when data are recorded in the recording film included in the information layer of the data-rewritable optical recording medium, a laser beam whose power is set to a recording power Pw having a sufficiently high level is irradiated on the recording film included in the information layer, and a region of the recording film where the laser beam is irradiated is heated to a temperature higher than the melting point of a phase change material and then melted. Subsequently, a laser beam whose power is set to a base power having a sufficiently low level is irradiated on the recording film included in the information layer, and a region of the melted recording film is rapidly cooled down. Consequently, the phase change material contained in the region of the recording film is changed from its crystalline state to an amorphous state, and the recording mark is formed in the recording film included in the information layer, so that data are recorded.
In the meantime, when the recording marks formed in the recording film included in the information layer of the data-rewritable optical recording medium are erased to erase data recorded in the recording film, a laser beam whose power is set to an erasing power Pe exceeding a base power Pb is irradiated on the recording film included in the information layer, regions where the recording marks of the recording film are heated to a temperature not less than the crystallizing temperature of a phase change material, and the regions where the recording film is heated are then slowly cooled down. Consequently, the phase change material contained in the regions of the recording film where the recording marks are formed is changed from its amorphous state to a crystalline state, and the recording marks are erased, thereby erasing the data.
Accordingly, by modulating the power of a laser beam irradiated on the recording film included in the information layer of the rewritable optical recording medium to a plurality of levels corresponding to the recording power Pw, the base power Pb, and the erasing power Pe, it is possible to directly overwrite data already recorded in the recording film with new data while erasing the recording marks formed in the recording film to form new recording marks as well as to form recording marks in the recording film included in the information layer to record data.
By doing so, when the data already recorded in the recording film included in the information layer of the rewritable optical recording medium is directly overwritten with new data, the phase change material in the amorphous state forming the recording marks is preferably crystallized to decrease the time required to erase the recording marks, and the recording film included in the information layer of the rewritable optical recording medium is usually formed of the phase change material having a high crystallization rate. By way of an example, a phase change material having an eutectic composition of Sb70Te30 is known as the phase change material having a high crystallization rate (See Patent Document 1).
[Patent Document 1] JP-A No.10-326436
However, when data are recorded in the optical recording medium including an information layer having a recording film formed of a phase change material and this optical recording medium is kept for a long time, it is difficult to crystallize the amorphized phase change material, which is known in the related art (For example, see T. Kikukawa, et. al., Jpn. J. Appl. Phys. Vol (41) 2002. pp 3020). Accordingly, it is difficult to erase the recording marks formed in the recording film as desired when data is directly overwritten with new data for the first time after an optical recording medium where the data is recorded is kept at a high temperature for a long time. Thus remaining recording marks not completely erased and newly formed recording marks are present together in the recording film after the already recorded data is directly overwritten with new data. As a result, jitter of reproduction signals degraded when the already recorded data is directly overwritten with new data to reproduce the recorded new data, and it is difficult to directly overwrite the data recorded in the recording film included in the information layer of the optical recording medium with new data as desired after the data is kept at a high temperature for a long time.
In addition, when the recording film formed of the phase change material is thinly formed, it is difficult to rapidly crystallize the amorphized phase change material, which is known in the related art (See N. Yamada, R. Kojima et. al., Technical Digest of ODS' 2001, p 22, 2001).
Accordingly, in an optical recording medium including an information layer having the thin recording film formed of the phase change material, it is very difficult to overwrite the data recorded in the recording film which is thinly formed with new data as desired after the data is kept at a high temperature for a long time.
However, in order to increase the recording capacity of the optical recording medium, an optical recording medium is proposed, which has a plurality of information layers each including a recording film formed of a phase change material, and data is recorded in the recording film included in an information layer farthest from a light incident plane among the information layers other than the information layer farthest from the light incident plane of a laser beam in such an optical recording medium, and when the data recorded in the recording film included in the information layer farthest from the light incident plane are reproduced, it is required to have a high optical transmittance to the laser beam so as to make the laser beam transmitted, so that the recording film included in the information layer needs to be made thin.